The present application relates to a cathode mixture, a non-aqueous electrolyte secondary battery, and its manufacturing method. More particularly, the invention relates to a non-aqueous electrolyte secondary battery using a cathode active material having an olivine type crystal structure.
In recent years, many portable electronic apparatuses such as camera integrated type VTR (Video Tape Recorder), cellular phone, laptop computer, and the like have been put into practical use and their miniaturization and light weights have been realized. Studies and developments to improve an energy density are vigorously being progressed with respect to batteries, particularly, secondary batteries as portable power sources of those electronic apparatuses. According to a lithium ion secondary battery among batteries using a non-aqueous electrolytic solution, since a larger energy density is obtained as compared with that of a lead battery or a nickel cadmium battery as an electrolytic solution secondary battery of an aqueous solution system in the related art, an expectation degree is large and their market is remarkably growing.
Particularly, in recent years, since features such as light weight and high energy density of the lithium ion secondary battery are suitable for uses of an electric automobile and a hybrid electric automobile, an examination to realize a large size and a high power of such a battery is vigorously being progressed.
In the lithium ion secondary battery for a general use or the like, generally, lithium cobalt acid LiCoO2 is mainly used as a cathode active material. However, there is a problem on a price and a supply amount from a viewpoint of an estimated amount of deposits of lithium cobalt acid. Therefore, such a tendency that a reasonable material in which anxiety about the supply amount is small is used is predicted.
Under such a situation, lithium iron phosphate LiFePO4 using reasonable iron whose production amount is large is highlighted. However, according to lithium iron phosphate, since an insertion/desorption reaction of lithium at the time of battery charge and discharge is slower and an electric resistance is larger as compared with those of lithium cobalt acid used in the related art, there is such a problem that in the charge and discharge of a large current, it is difficult to obtain a sufficient charge/discharge capacitance in association with an increase in overvoltage.
With respect to such a problem, various countermeasures have been taken and, for example, the following methods (1) to (4) have been proposed.
(1) A particle diameter of an active material is reduced and a specific surface area is increased.
(2) A conductive assistant such as carbon or the like is held on the particle surface of the active material.
(3) When a cathode mixture is manufactured, carbon black, fibrous carbon, or the like is added.
(4) An adhesive force of component members is improved by using a binder having a large binding force.
Specifically speaking, the above methods (1) to (4) have been disclosed in following Patent Documents 1 to 4.
Patent Document 1: JP-A-2002-110162
Patent Document 2: JP-A-2001-110414
Patent Document 3: JP-A-2003-36889
Patent Document 4: JP-A-2005-251554
(1) In Patent Document 1, there has been disclosed such a technique that a particle diameter of the primary particle of lithium iron phosphate is limited to 3.1 μm or less and the specific surface area of the cathode active material is sufficiently increased, thereby increasing an electronic conductivity in the cathode.
(2) In Patent Documents 2 and 3, such a technique that a conductive microparticle is held on the particle surface of lithium iron phosphate and the active material is improved, thereby raising the charge/discharge capacitance in the charge/discharge of the large current has been disclosed.
(3) To reduce the electric resistance of the cathode, powdered carbon such as carbon black, flake carbon such as graphite, or fibrous carbon is generally mixed.
(4) In Patent Document 4, there has been disclosed such a technique that by using a binder having a large binding force, an adhesion between the cathode active material and the conductive assistant, an adhesion between the cathode active material and a collector, and an adhesion between the collector and the conductive assistant are improved, thereby improving characteristics at the time of the large-current charge/discharge.
The electric resistance of the cathode decreases by the foregoing methods of (1) to (4). However, to adapt to the battery for a high-power application which is used in the electric automobile, a hybrid automobile, or the like, according to lithium iron phosphate having the olivine type crystal structure, it is still difficult to obtain sufficient output characteristics as compared with those of lithium cobalt acid having a stratified structure or those of lithium manganese acid having a spinel structure.
It is also understood that if the foregoing methods of (1) and (3) are used, another problem in which life characteristics deteriorate at a high temperature also occurs.